This invention relates to high pressure sodium vapor lamps and, more particularly, to a lamp that includes improved means for compensating for the tendency of the voltage across the arc tube of the lamp to increase over the lamp life due to elevated "cool" spot temperature. Two factors in particular cause voltage rise in the lamp. The more prevalent factor is a gradual increase in "cool" spot temperature which controls lamp vapor pressure to a large degree and therefore the voltage. The second factor is a gradual loss of sodium due to reaction with the lamp components or by diffusion through the seal. Voltage rise is undesirable, because when the voltage increases to a certain value the lampe will extinguish.
One such compensating means disclosed in U.S. Pat. No. 3,851,207 dated Nov. 26, 1964 issued to McVey, comprises a high-intensity sodium vapor arc lamp utilizing a slender tubular envelope of alumina ceramic having self-heating electrodes mounted at opposite ends and containing an excess pool of sodium mercury amalgam at the "cool" spot in the lower end. The electrode at the lower end includes a resistive heater portion and lamp current flowing therethrough provides supplementary heat to the amalgam pool varying according to lamp current. This design counters the tendency to arc voltage rise with life and reduces the sensitivity of the lamp to radiant energy reflected back from the fixture.